It depends on what you mean by 'legacy code'. An existing C++ codebase can be used for a Windows Store app, but there are two things to keep in mind:

(a) It needs to be recompiled using VS 2012. This is because only the VS 2012 CRT is supported for Windows Store apps. You can't deploy or use older versions of the VC++ runtime (VS 2010, 2008, 2005, etc.)

(b) The resulting code must still pass the WACK tool. That means it can only use approved Win32 APIs that are supported for use in Windows Store apps. Generally speaking, you can't just take an existing static library or DLL built with some other toolset
that ran fine on Windows 7 and just drop it directly into a Windows Store app project because it is likely to fail this requirement.

There is certainly plenty of existing C++ code libraries that can be made to work in a Windows Store app, but at a minimum will require recompilation.

It depends on what you mean by 'legacy code'. An existing C++ codebase can be used for a Windows Store app, but there are two things to keep in mind:

(a) It needs to be recompiled using VS 2012. This is because only the VS 2012 CRT is supported for Windows Store apps. You can't deploy or use older versions of the VC++ runtime (VS 2010, 2008, 2005, etc.)

(b) The resulting code must still pass the WACK tool. That means it can only use approved Win32 APIs that are supported for use in Windows Store apps. Generally speaking, you can't just take an existing static library or DLL built with some other toolset
that ran fine on Windows 7 and just drop it directly into a Windows Store app project because it is likely to fail this requirement.

There is certainly plenty of existing C++ code libraries that can be made to work in a Windows Store app, but at a minimum will require recompilation.